Setup Separate Root level Sharepoint sites on Office 365?

We are in the process of Migrating from On-Premise Exchange and Sharepoint to Office 365 with Sharepoint.

We currently have an On-Premise Sharepoint 2013 Servers. We host Mail and Sharepoint for a few Companies, so the Sharepoint server is home to several Root level Sites:

Each root site is Managed by their own Administrators and the users are different for each site.
All users are part of the same Activity Directory Domain.
I have all of the domains Validated in Office 365 and mail is flowing to all of them.

I just dont see where I can Add a new Root level Domain  and separate site for each of the existing sites I have.
Scott TownsendIT DirectorAsked:
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Vasil Michev (MVP)Commented:
In SPO you can only use custom domains for the Public website, which is actually deprecated now. All other Site Collections will have to be created under - no way to add other domains.

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Scott TownsendIT DirectorAuthor Commented:
I'm guessing I cannot setup Multiple Linked Tenants that all use the same Office 365 Subscriptions/Users/AD that have access to SharePoint?

We have Server Admins that Manage all of the sites and then there are the Specific employees that have access to the individual sites, so I for instance have access to all sites. My Manager has access to 3 of the 4 another employee has access to 2 and another employee has access to just the one. The companies don't need to see the other sites that they don't have access to at all. So I don't want them to to go to the root site and try to authenticate and fail because they think its their site.
Vasil Michev (MVP)Commented:
You can have them all in the same tenant, but you have to properly permission each site collection and sites below SCs. And all site collections will have the same domain name.

Then if you add Groups/Teams in the mix, it gets more complicated, as each of these gets its own Site collection...
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